Continuous preparation of silicon nitride powder

ABSTRACT

Silicon nitride powder is continuously prepared by feeding metallic silicon powder into a fluidized bed comprising silicon nitride powder and nitrogen or ammonia gas and discharging a nitrided product from the fluidized bed. The metallic silicon powder is pretreated at a temperature of 1,000 DEG -1,400 DEG  C. under a vacuum of 0.001-100 Torr before it is subject to nitriding reaction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for continuously preparing siliconnitride powder having a high alpha-type content at a high percentconversion.

2. Prior Art

One typical prior art process for continuously preparing silicon nitridepowder is by direct nitriding through a fluidized bed. JP-B 6482/1994 bythe same assignee as the present invention discloses a two-stagereaction process comprising the steps of continuously feeding a stockmaterial to be nitrided containing metallic silicon powder into a firstfluidized bed comprising silicon nitride powder and a non-oxidizing gascontaining nitrogen gas or ammonia gas where primary nitriding reactiontakes place and then feeding the product into a second fluidized bedcomprising silicon nitride powder and a non-oxidizing gas containingnitrogen gas or ammonia gas where secondary nitriding reaction ofnitriding the unreacted stock material takes place.

In this process, however, the stock material feed to be nitrided canpass shortcut since the fluidized bed is a fully mixed system. Then theresidence time is short and the output is a silicon nitride powdercontaining a large amount of unreacted silicon. As a result, it is verydifficult to produce silicon nitride powder at a conversion as high as95% or more. In order that silicon nitride powder be produced at a highpercent conversion by this continuous process, it is recommended (1) toextend the residence time, (2) to add an additional stage or stages, (3)to add a catalyst, and (4) to elevate the reaction temperature.

These countermeasures, however, raise problems. In the event ofextending the residence time (1) or using multiple stages (2), theresidence time through the production system is increased at thesacrifice of productivity. Where a catalyst is added (3), the nitrideproduct can be contaminated with impurities. Where the reactiontemperature is elevated (4), beta-type silicon nitride which is stableat elevated temperatures is produced in a higher proportion, resultingin a drop of sintered strength. From an operational aspect, siliconmonoxide deposits on the inner wall of an output duct for dischargingthe nitride product. After long-term operation, the output duct isnarrowed to such an extent that the product cannot be eventuallydischarged.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a processfor continuously preparing high alpha-type silicon nitride powder at ahigh conversion within a short time in an efficient and commerciallyadvantageous manner.

The present invention provides a process for continuously preparing highalpha-type silicon nitride powder by continuously feeding metallicsilicon powder into a fluidized bed comprising silicon nitride powderand a non-oxidizing gas containing nitrogen gas or ammonia gas, andcontinuously discharging a nitrided product from the fluidized bed.According to the feature of the invention, the metallic silicon powderas a feed stock is pretreated at an elevated temperature under vacuumbefore it is subject to nitriding in the fluidized bed. Then siliconnitride powder having an alpha-type content of at least 90% can beproduced at a conversion of at least 95% within a short time in anefficient manner.

Making investigations in search of a process capable of preparing highalpha-type silicon nitride powder having an alpha-type content of atleast 90% at a conversion of at least 95% without lowering productivitywhile eliminating the drawback of the above-mentioned continuoustwo-stage fluidized bed system that reactivity lowers as a result ofshortcut passage, we reached the conclusion that increasing a rate ofreaction, especially at the initial of nitriding is effective. Thus westudied the reaction mechanism and made a series of experiments foraccelerating the rate of reaction. We confirmed that even after anitriding temperature is reached, reaction does not take place for acertain time (a delay in rise time) and that after the start ofreaction, reaction proceeds with concomitant ablation or stripping ofsilicon nitride coating formed on the surface of metallic silicon (seeY. Inomata and Y. Uemura, YogyoKyokai-shi, 83, 244 (1975)). We got theforecast that the rate of reaction can be significantly increased byestablishing the means or formulation for eliminating a delay in risetime and promoting the stripping of silicon nitride coating on thesurface of metallic silicon. It is believed that the delay in rise timecorresponds to the time taken to remove autoxidation film formed on thesurface of metallic silicon, and the stripping of silicon nitridecoating takes place under the impetus of differential thermal expansionbetween metallic silicon and silicon nitride and the vapor pressure ofmetallic silicon vapor or the like given off from the metallic siliconsurface. Then the delay in rise time can be effectively eliminated bypreviously removing oxide coating on the metallic silicon surface. Withrespect to the promotion of stripping of silicon nitride coating, sincethe bond energy (812 kJ/mol) of Si--O is greater than the bond energy(320 kJ/mol) of Si--N by a factor of more than 2, the absence of oxygenallows for more smooth stripping of silicon nitride coating and hence,more brief production of silicon nitride powder at a higher percentconversion or nitridation. From the operational aspect, it was foundthat oxygen in stock material is involved in the following set ofreactions:

    Si+SiO.sub.2 →2SiO

    SiO.sub.2 +H.sub.2 →SiO+H.sub.2 O

    SiO.sub.2 →SiO+1/2O.sub.2

to form silicon monoxide gas which deposits on the inner wall of anoutput duct to narrow the output duct, obstructing long-term continuousoperation. That is, from both the aspects of reaction and operation,oxygen present on the surface of metallic silicon stock material andtaken into metallic silicon upon exposure to elevated temperatures has adeleterious influence. In order to attain the object of the invention,it is essential to establish a method of efficiently removing oxygenfrom metallic silicon. Making further investigations on the method ofefficiently removing oxygen from metallic silicon, we have found thatthe oxygen content of metallic silicon powder stock material can bereduced by pretreating the stock material at an elevated temperature invacuum. By continuously feeding the pretreated stock material into afluidized bed comprising silicon nitride powder and a non-oxidizing gascontaining nitrogen gas or ammonia gas, silicon nitride powder can beproduced at a high percent conversion within a short time. Additionally,from the operational aspect, the reduced oxygen content of metallicsilicon powder stock material suppresses the generation of siliconmonoxide gas through the above-mentioned set of reactions to drasticallyreduce the deposition of silicon monoxide on the output duct inner wall,resulting in a significant improvement in operational stability.

Accordingly, the present invention provides a process for continuouslypreparing high alpha-type silicon nitride powder comprising the steps ofpretreating metallic silicon powder at an elevated temperature undervacuum, continuously feeding the pretreated metallic silicon powder intoa fluidized bed comprising silicon nitride powder and a non-oxidizinggas containing nitrogen gas or ammonia gas, and continuously discharginga nitrided product from the fluidized bed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features of the present invention will be apparentwith reference to the following description and drawings, wherein:

the only figure, FIG. 1 is a schematic illustration of a silicon nitrideproducing system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The process of the present invention starts with metallic silicon powderas a stock material. Though not critical, it is preferred to usemetallic silicon powder of 150 mesh under, preferably 325 mesh under asa stock material. Alternatively, metallic silicon powder is granulatedand consolidated into particles having a mean particle size of 100 μm to10 mm, especially 300 μm to 1 mm. Such granulation and consolidation maybe carried out by adding a binder such as polyvinyl alcohol to metallicsilicon powder, granulating the mixture into granules, and brieflyfiring the granules at a temperature of about 1,000° to 1,400° C. suchthat the silicon granules are joined to each other, but not fused. Forpromoting nitriding reaction, suitable additives such as K, Na, Cu, Li,V, Ca and Fe or compounds thereof may be added to metallic siliconpowder.

According to the present invention, metallic silicon powder ispretreated at an elevated temperature under vacuum before it is subjectto nitriding reaction. Where granulated and consolidated metallicsilicon powder is used as a stock feed to be nitrided, metallic siliconpowder may be pretreated at an elevated temperature in vacuum and thendisintegrated to a size of 100 μm to 10 mm before use. Alternatively,untreated metallic silicon powder may be granulated and consolidated andthen pretreated at an elevated temperature in vacuum before use. In onepreferred embodiment, however, when metallic silicon powder isgranulated and consolidated, the firing for consolidation is carried outat an elevated temperature in vacuum.

For the pretreatment, any desired equipment may be used. A batch furnacewith least leakage is preferred. By means of a vacuum pump, the furnaceis preferably evacuated to a vacuum of 10^(-') to 100 Torr, especially10⁻² to 50 Torr. Pretreatment under a vacuum of higher than 100 Torrwould be ineffective. Under a vacuum of lower than 10⁻³ Torr, noadditional advantage would be obtained and metallic silicon can be lostthrough evaporation, resulting in a lower yield. The treatingtemperature is preferably 1,000° to 1,400° C., more preferably 1,200° to1,350° C. Pretreatment at a temperature of lower than 1,000° C. would beineffective whereas metallic silicon can be melted at a temperature ofhigher than 1,400° C., detracting from reactivity.

According to the present invention, the metallic silicon powder whichhas been pretreated at an elevated temperature under vacuum iscontinuously fed into a fluidized bed comprising silicon nitride powderand a non-oxidizing gas containing nitrogen gas or ammonia gas wherenitriding reaction takes place. The resulting nitride product iscontinuously discharged from the fluidized bed. The reaction conditionsof the fluidized bed may be well-known ones. More particularly, thefluidized bed may be set at a temperature of 1,000° to 1,500° C.,preferably 1,200° to 1,400° C. At temperatures below 1,000° C., littlenitride would form on the surface of metallic silicon particles.Temperatures above 1,500° C. can melt metallic silicon to impedereaction. The non-oxidizing gas serving to form the fluidized bed maycontain 10 to 100% by volume, especially 60 to 90% by volume of nitrogengas or ammonia gas. Optionally, the non-oxidizing gas contains hydrogengas and an inert gas such as helium and argon.

In the practice of the invention, the nitriding reaction in thefluidized bed is preferably one-stage reaction although two- ormulti-stage reaction is employable if desired.

Referring to FIG. 1, there is schematically illustrated an exemplaryreaction system which is used in the practice of the invention. Thesystem includes a reactor 1 in the form of a vertical cylindricalvessel. The reactor 1 is provided with a gas dispersing perforated plate2 at a lower position, above which a fluidized bed 3 is formed and belowwhich a reaction gas feed compartment 8 is defined. A heater 4 isdisposed outside the reactor 1 so as to circumscribe a fluidizedbed-forming region. The reactor 1 is provided at a lower end with a gasinlet 5 which is connected to a gas blender 7 through a gas feed conduit6. The gas blender 7 receives nitrogen gas and hydrogen gas, forexample, mixes them, and then supplies the gas mixture into the reactiongas feed compartment 8 through the gas feed conduit 6 and the gas inlet5. The gas mixture is dispersed into the fluidized bed-forming regionthrough apertures in the plate 2 to form the fluidized bed 3 with thesilicon nitride powder which is previously introduced therein. A feedpipe 9 for feeding a stock material is tightly passed through the top ofthe reactor 1 and extended through the reactor 1 until the lower end ofthe feed pipe 9 reaches a lower portion of the fluidized bed 3. The feedpipe 9 at the upper end is connected to a screw feeder 11 which is, inturn, connected to a hopper 10. An output pipe 12 for dischargingsilicon nitride is also tightly passed through the bottom of the reactor1 and extended through the reactor 1 until the upper end of the outputpipe 12 reaches near the upper surface of the fluidized bed 3. Theoutput pipe 12 at the lower end is connected to a silicon nitridecollector 13. In operation, the hopper 10 receives metallic siliconpowder stock which has been pretreated at an elevated temperature invacuum and delivers it to the screw feeder 11. The stock material isthen fed into the fluidized bed 3 at a lower position through the feedpipe 9. In the fluidized bed 3, the stock material is subject tonitriding reaction. The resulting silicon nitride powder is dischargedinto the output pipe 12 at the upper end of the fluidized bed 3 andcollected in the collector 13.

EXAMPLE

Examples of the present invention are given below by way of illustrationand not by way of limitation. All parts are by weight.

Examples 1-5 and Comparative Examples 1-4

Silicon nitride powder was continuously prepared using a continuousfluidized bed reactor system as shown in FIG. 1. The stock material usedherein was prepared by adding 1% by weight as solids of polyvinylalcohol to metallic silicon powder having a mean particle size of 4.5μm, milling the mixture, granulating the mixture into granules having amean particle size of 0.5 mm by means of a granulating extruder, andcalcining them under the conditions shown in Table 1. The hopper 10 wascharged with this stock material. The reactor 1 having an inner diameterof 250 mm was charged with 25 kg of silicon nitride powder while a gasmixture consisting of 150 normalized liter/min. of nitrogen gas and 40normalized liter/min. of hydrogen gas mixed in the gas blender 7 wasintroduced in the reaction gas feed compartment 8 from the gas inlet 5and then dispersed into the center region of the reactor through the gasdispersing plate 2 to form the fluidized bed 3. The heater 4 is actuatedto heat the fluidized bed 3 at 1,350° C. The stock material wasintroduced deeply into the fluidized bed 3 at a rate of 4 kg/hourthrough the feed pipe 9. Silicon nitride powder formed as a result ofnitriding reaction in the fluidized bed was discharged into the outputpipe 12 in an overflowing manner and recovered in the collector 13.

In this way, the fluidized bed reactor system was continuously operatedfor 200 hours. Using X-ray diffraction and analysis instruments, thesilicon nitride powder recovered was measured for a percent conversionand alpha-type content. The weight of SiO deposited on the inner wall ofthe output pipe 12 was measured. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Pretreatment    Silicon nitride powder                                                                        SiO                                           Temp.     Vacuum    Conversion                                                                              α content                                                                       deposit                                 (°C.)                                                                            (Torr)    (%)       (%)     (g)                                     ______________________________________                                        E1    1300    1 × 10.sup.-1                                                                     97.6    93.5    trace                                 E2    1300    1 × 10.sup.-3                                                                     97.3    94.2    trace                                 E3    1300     50       96.1    93.3    trace                                 E4    1000    1 × 10.sup.-1                                                                     95.5    92.3    trace                                 E5    1400    1 × 10.sup.-1                                                                     96.8    93.2    trace                                 CE1   1300    1 atm.    83.0    90.7    220.5                                 CE2   1450    1 × 10.sup.-1                                                                     45.3    95.2    trace                                 CE3    900    1 × 10.sup.-1                                                                     88.5    93.8     30.2                                 CE4   1300    150       92.5    92.7     16.8                                 ______________________________________                                    

It has been described that by pretreating metallic silicon powder at anelevated temperature in vacuum, there is obtained highly reactive stockmaterial. Using the pretreated stock material, continuous nitridingreaction in a fluidized bed can be carried out without a substantialdrop of percent conversion which is otherwise caused by shortcutpassage. Then the process is successful in producing silicon nitridepowder at a high percent conversion within a short time and in anefficient stable manner without system trouble such as blockage of anoutput pipe.

Japanese Patent Application No. 345247/1995 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in the light of theabove teachings. It is therefore to be understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

We claim:
 1. A process for continuously preparing high alphatype siliconnitride product comprising the steps of:pretreating metallic siliconpowder at 1000° to 1400° C. under 10⁻³ to 100 Torr to reduce the oxygencontent of the metallic silicon powder before it is subject to anitriding reaction, continuously feeding the pretreated metallic siliconpowder into a fluidized bed comprising silicon nitride powder and anon-oxidizing gas containing nitrogen gas or ammonia gas, andcontinuously discharging a nitrided product from the fluidized bed. 2.The process of claim 1 further comprising the step of granulating thepretreated metallic silicon powder into granules having a mean particlesize of 100 μm to 10 mm before it is fed into the fluidized bed.
 3. Aprocess for continuously preparing high alphatype silicon nitrideproduct comprising the steps of:granulating metallic silicon powder intogranules having a mean particle size of 100 μm to 10 mm, pretreating thegranules at 1000° to 1400° C. under 10⁻³ to 100 Torr to reduce theoxygen content of the granules before they are subject to a nitridingreaction, continuously feeding the granules into a fluidized bedcomprising silicon nitride powder and a non-oxidizing gas containingnitrogen gas or ammonia gas, and continuously discharging a nitridedproduct from the fluidized bed.